The present invention relates generally to the field of wireless transmitters and, particularly, to suppression tuning of carrier signals transmitted by a radio telephone.
A quadrature modulator is the combination of two mixers and a quadrature phase shift block. This combination forms a structure that allows the creation of linear modulation when excited by a signal that has been split into its quadrature components. The quadrature components of the modulating signal are the in-phase (xe2x80x9cIxe2x80x9d) signal and the quadrature (xe2x80x9cQxe2x80x9d) signal. The outputs of the two mixers are summed (xe2x80x9cxcexa3xe2x80x9d) to produce the overall modulator output.
Many digital radio transceivers of wireless communication devices utilize a quadrature modulator in the transmission path to generate the desired radio frequency output signal. A quadrature modulator is the only practical approach for wireless communication device designed to operate in any digital wireless communication systems, such as IS136, PDC, CDMA and EDGE, which utilize a modulation type requiring a non-constant transmit envelope. For constant envelope systems, such as GSM and AMPS, a quadrature modulator is often used because it represent a practical approach to the modulation problem, particularly when a device is intended for use in multiple systems.
The problems with quadrature modulators are due to the fact that they are not perfect. They suffer various impairments that ultimately impact the performance of the transmitter, such as carrier feedthrough. Carrier feedthrough arises primarily because of DC offsets in the various circuits that constitute the modulator circuit. These circuits include the digital-to-analog (xe2x80x9cD/Axe2x80x9d) converter that generates the modulator input as well as any filters after the converter, the individual devices in the IQ mixer, and the devices that provide the local oscillator (xe2x80x9cLOxe2x80x9d) signal to the mixers.
Several approaches to the above problem specify the performance of both the D/A converter and the quadrature modulator as tightly as possible. These approaches work well for many systems, although there is a measurable yield hit at the integrated circuit test level. However, the radio level requirement for carrier feedthrough is significantly more stringent for future systems.
In any analog implementation of a quadrature modulator, non-ideal conditions exist with respect to amplitude balance, phase accuracy and DC offsets. Several techniques exist for dealing with amplitude balance and phase accuracy, but a viable, inexpensive technique for dealing with DC offsets is still needed and desired. DC offset in the I path is equal to the ratio of the absolute DC offset to the peak value of the I baseband signal, and the DC offset in the Q path is equal to the ratio of the absolute DC offset to the peak value of the Q baseband signal.
Radio transceivers that correct amplitude and/or phase errors between I and Q branches of a quadrature modulator are generally known in the art. For example, U.S. Pat. No. 5,933,448 to K. Katisko titled Automatic Tuning of a Radio Transceiver describes a transmission signal is sampled and directed to a TRX loop mixer and, then, directed to a reception branch for Received Signal Strength Indicator (xe2x80x9cRSSIxe2x80x9d) calculation. The existing circuitry of a radio transceiver requires the addition of a loop mixer, a local oscillator and a pair of directional couplers to implement the correction scheme of this patent.
Also, U.S. Pat. No. 5,371,481 to E. Tiittanen, et al. titled Tuning Technique For I/Q Channel Signals in Microwave Digital Transmission Systems describes a vector modulation system that compensates baseband magnitude and phase errors. A filter power meter, such as a spectrum analyzer, that is external to the system and power control unit is connected to an antenna to measure the narrow band RF-power. The system compensates the baseband magnitude and phase errors based on the difference between the desired sideband and the undesired sideband. The existing circuitry of a radio transceiver must be connected to an external power meter and measurement processor to implement the correction scheme of this patent.
In view of the above, there is a need for a viable technique for controlling carrier feedthrough or, more particularly, dealing with DC offset problems. The cost of implementing the technique must be inexpensive so that it may remain viable. Also, the technique should not require external test equipment and should be relatively fast. Thus, the technique should be implemented using as many existing components of a typical wireless communication device as possible. In addition, the above technique must maintain the stringent radio level requirements for carrier feedthrough for present as well as future systems.